x86: use FDT accessors for FDT blob header data

Remove the direct accesses to FDT header data using accessor
function instead. This makes the code more readable and makes the FDT
blob structure more opaque to the arch code. This also prepares for
removing struct boot_param_header completely.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Tested-by: Grant Likely <grant.likely@linaro.org>